The present invention relates to electrorheological fluids which contain as the dispersed particles salts of polymers, and electrorheological devices made using such fluids.
Electrorheological ("ER") fluids are fluids which can rapidly and reversibly vary their apparent viscosity in the presence of an applied electric field. ER fluids are generally dispersions of finely divided solids in hydrophobic, electrically non-conducting oils. They have the ability to change their flow characteristics, even to the point of becoming solid, when subjected to a sufficiently strong electrical field. When the field is removed, the fluids revert to their normal liquid state. ER fluids may be used in applications in which it is desired to control the transmission of forces by low electric power levels, for example, in clutches, hydraulic valves, shock absorbers, vibrators, or systems used for positioning and holding work pieces in position.
ER fluids have been known since 1947, when U.S. Pat. No. 2,417,508 was issued to Winslow, disclosing that certain dispersions of finely divided solids such as starch, carbon, limestone, gypsum, flour, etc., dispersed in a non-conducting liquid would undergo an increase in flow resistance when an electrical potential difference was applied. In the extensive work which has followed this discovery, many variations of ER fluids have been discovered, in which the solid phase, the liquid phase, or other components have been varied. One feature of most ER fluids is that at least a small amount of a polar substance, generally water, must be absorbed or adsorbed by the dispersed particles in order to provide significant ER properties. Unfortunately, water-containing systems generally exhibit limited useful operating temperature ranges. At temperatures above about 100.degree. C. the performance of such systems typically deteriorates due to volatilization of the water.
Among the various attempts to provide an improved ER fluid are the following:
U.S. Pat. No. 4,033,892 discloses electrorheological fluids wherein the solid substance is a polyhydric alcohol which contains acid groups and which has an open structure wherein a significant amount of water is absorbed. In a preferred embodiment the polyhydric alcohol is a polymer of a monosaccharide which is insoluble in water. Other suitable materials include polyvinyl alcohol and polymers of a monosaccharide derived from starch. The polyhydric alcohol may be a salt rather than a free acid. ER fluids which contain a relatively low amount of absorbed water may be particularly useful for high temperature applications.
U.S. Pat. No. 4,473,778 discloses an electroviscous fluid comprising water-containing particles of a phenolformaldehyde polymer dispersed in a non-conducting liquid. In a preferred embodiment the polymer comprises the dilithium salt of 2,2',4,4'-tetrahydroxybenzophenone condensed with formaldehyde.
U.S. Pat. No. 4,812,251 discloses an electrorheological fluid comprising a hydrophilic solid and a hydrophobic liquid component. This reference reports that ionic polymers, such as algenic acid, polymethacrylic acid, and phenol-formaldehyde resins have been used, usually as salts. The solid component can comprise an organic polymer containing free or salified acid groups.
U.S. Pat. No. 4,992,192 discloses electrorheological fluids prepared from monomers (such as styrene or methacrylic acid) polymerized by dispersion polymerization in a medium which also serves as the dispersion medium for the fluid. The particles are modified by polymerizing a hydrophilic shell around the particle followed by neutralization through addition of an organic soluble base. Suitable monomers for the hydrophilic shell include maleic acid, vinyl toluene sulfonate, and others. The hydrophilic shell polymer is neutralized by reaction with e.g. butyl lithium.
The present invention now provides an ER fluid which is based on a polymeric salt which retains its useful function at elevated temperatures.